MARKET DISEASES OF VEGETABLES. 45 



Deep scab is an advanced stage of common scab. Often 

 insects or mites live in scab pockets. It is not known to just 

 what extent these are responsible for the Lesions. 



Common scab occurs in all varieties of potatoes and is 

 most common in potatoes grown in alkaline soil. Treatment 

 of seed stock with corrosive sublimate, and planting in unin- 

 fected soil or acid soil are fair preventives of the trouble. 



Wire worm or grub injury often are confused with scab 

 lesions. It frequently is difficult to differentiate between 

 them. Wireworm injury is generally marked by extensive 

 channeling in the tuber skin, while grub injury is marked by 

 deep, broad pits with protruding or overhanging rims. Wire- 

 worm and grub injuries often are points of entry for Fusa- 

 rium species. 



Scab and insect injuries depreciate the value of tubers 

 for table use by making them unsightly, and by necessitat- 

 ing deep paring with attendant loss of food. 



Ref. (10); (38); (39); (50). 



POTATO: POWDERY SCAB. 



Cause: A slime mold (Spongospora subterranea). 



In the early stages powdery scab consists of pimple or 

 blister-like eruptions, one-sixteenth to one-quarter inch in 

 diameter, which are completely covered by the skin of the 

 tuber. Later these coverings rupture and expose pits, which 

 are single or joined, and fringed by the flaring, papery, 

 torn, and more or less toothed remnants of the skin. The 

 interior of these pits is at first filled with a brown powder 

 or dust, which is usually absent when the tubers reach mar- 

 ket. Often the tissues surrounding the pits become discol- 

 ored and sunken, and each pit or group of pits appears as a 

 crater in a sunken zone. 



Powdery scab differs from common scab in the more 

 nearly circular lesions, smaller pits, presence of brown dust, 

 star-like rupture of the skin, and relative absence of cork 

 formation. 



This disease is not as serious a menace as it was once 

 thought to be. It seems to be localized in a few sections of 

 North America, and is causing little damage. It is favored 

 by cool, moist weather. 



The causal organism is introduced into the soil with dis- 

 eased seed stock, and tuber infection occurs in the field. 



Affected stock is fit for table use. Its market value, how- 

 ever, is decreased because of its unsightliness, and because 

 of the waste due to deep paring. 



Ref. (44); (46). 



POTATO: LATE BLIGHT TUBER ROT. 



Cause: A fungus (Phytophthora infestans). 



Late blight tuber rot is characterized externally by de- 

 pressed, discolored areas of irregular but definite outline. 

 These may occur merely as spots or as very extensive lesions. 



